Use of melatonin for induction of general anesthesia

ABSTRACT

Melatonin (N-acetyl-5-methoxytryptamine), or its biologically active analogues, are used to induce anesthesia.

CROSS REFERENCE TO A RELATED APPLICATION

[0001] This application is a continuation-in-part of U.S. Ser. No.60/233,785 filed Sep. 19, 2000 and claims the benefit of that filingdate.

BACKGROUND OF THE INVENTION

[0002] In the medical field there is a continuing need for new compoundshaving demonstrated use for inducing anesthesia. It is not onlyimportant to induce beneficial anesthesia, but it must be done in amanner that limits toxicity to patients, and as well, minimizes what isknown as “anesthesia hangover”.

[0003] The pineal hormone melatonin (N-acetyl-5-methoxytryptamine) hasseveral putative functions, including regulation of circadian rhythms,regulation of the reproductive axis and antioxidant activity.Autoradiographic studies and receptor assays have demonstrated thepresence of melatonin receptors in various regions of the centralnervous system and in other tissues in humans.

[0004] Exogenous administration of melatonin has been found by severalinvestigators to facilitate sleep onset and improve quality of sleep.Available data suggest that the sleep-inducing properties of melatoninmay differ from those of benzodiazepines. Benzodiazepines decreaseduration of REM sleep after single administration of a high dose orlong-term administration of low dose. Benzodiazepines also reduceslow-wave sleep, thus negatively influencing sleep quality. In contrast,a single low dose of melatonin produced no suppression of REM sleep.Furthermore, unlike benzodiazepines, melatonin does not induce“hangover” effects.

[0005] In a previous publication of the author, British Journal ofAnesthesia 82(6):875-80(1999), low-level dosing of oral melatonin in asublingual fashion was demonstrated as an effective pre-medication,prior to administering a general anesthetic. Patients who wereadministered such low-level doses sublingually had a significantdecrease in anxiety levels and an increase in levels of sedation beforeoperation. However, as pointed out in that article, the use of melatoninin anesthesia had as of then never been evaluated properly, and to theinventor's present knowledge it has never been used as a generalanesthetic.

[0006] This invention has as its primary objective the development ofpineal hormone melatonin (N-acetyl-5-methoxytryptamine) or itsbiologically active analogues as a general anesthetic which can be usedwithout any significant anesthetic hangover. The continuing need in theart for meeting such an objective is readily apparent.

SUMMARY OF THE INVENTION

[0007] Anesthetic compositions are prepared using apharmaceutically-acceptable carrier and an anesthetic-inducing effectiveamount of melatonin or biologically active analogues of melatonin. Theinvention also relates to the method of administration using thedescribed compositions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0008] N-acetyl-5-methoxytryptamine (melatonin) is synthesized mainly bythe pineal gland, and to a lesser extent by extra pineal tissues such asthe retina, hardening gland, and gastrointestinal tract. Melatonin hasthe following structure:

[0009] As seen, the chemical formula for melatonin isN-acetyl-5-methoxytryptamine. From time to time in the specificationapplicant uses the term “N-acetyl-5-methoxytryptamine (melatonin), orits biologically active analogues”. As used herein, this phrase refersto the precise compound itself and other compounds having the samegeneral structure, but only differing in minor moieties, and thereforestill having the same biological activity of anesthetic-inducingeffectiveness. The biologically active compound of the presentinvention, such as melatonin, may be derived or extracted from thepineal gland, or it can be synthesized from 5-Methoxyindol as a startingmaterial by known routes, Szmuszkovicz et al., J. Org. Chem. 25, 857(1960). Biochemical role of melatonin: Chem. & Eng. News 45, 40 (May 1,1967).

[0010] The anesthetic active, i.e., the N-acetyl-5-methoxytryptamine(melatonin), or its biologically active analogues, can be administeredwith traditionally acceptable pharmaceutical carriers. Examples includeIntralipid®, Cyclodextrin, and others, some of which are brieflyhereinafter described. However, there is no need for detaileddescription of suitable anesthetic carriers because they are so wellknown in the industry.

[0011] The composition may be administered by conventionaladministration methods for anesthetics, i.e., oral administration, nasalrespiratory administration, bolus injection, intravenous administrationby repeated doses or by continuous infusion, rectal, vaginal,sublingual, cutaneous and slow release routes. It may be, and often ispreferred, that it be administered in two or more ways, such as by bolusinjection followed by continuous intravenous administration.

[0012] The liquid forms in which the novel compositions of the presentinvention may be incorporated for administration orally or by injectioninclude aqueous solutions, suitably flavored syrups, aqueous or oilsuspensions, and flavored emulsions with edible oils such as cottonseedoil, sesame oil, coconut oil or peanut oil, as well as elixirs andsimilar pharmaceutical vehicles. Suitable dispersing or suspendingagents for aqueous suspensions include synthetic and natural gums suchas tragacanth, acacia, alginate, dextran, sodium carboxymethylcellulose,methylcellulose, polyvinylpyrrolidone or gelatin.

[0013] Preferred compositions for administration by injection includethose comprising a melatonin biologically active analogue as the activeingredient, in association with a surface-active agent (or wetting agentor surfactant) or in the form of an emulsion (as a water-in-oil oroil-in-water emulsion).

[0014] Suitable surface-active agents include, in particular, nonionicagents, such as polyoxyethylenesorbitans (e.g. Tween™ 20, 40, 60, 80 or85) and other sorbitans (e.g. Span™ 20, 40, 60, 80 or 85). Compositionswith a surface-active agent will conveniently comprise between 0.05 and5% surface-active agent, and preferably between 0.1 and 2.5%. It will beappreciated that other ingredients may be added, for example mannitol orother pharmaceutically acceptable vehicles, if necessary.

[0015] Suitable emulsions may be prepared using commercially availablefat emulsions, such as Intralipid™, Liposyn™, Infonutrol™, Lipofundin™and Lipiphysan™. The active ingredient may be either dissolved in apre-mixed emulsion composition, or alternatively it may be dissolved inan oil (e.g. soybean oil, safflower oil, cottonseed oil, sesame oil,corn oil or almond oil) and an emulsion formed upon mixing with aphospholipid (e.g. egg phospholipids, soybean phospholipids or soybeanlecithin) and water. It will be appreciated that other ingredients maybe added, for example glycerol or glucose, to adjust the tonicity of theemulsion. Suitable emulsions will typically contain up to 20% oil, forexample, between 5 and 20%. The fat emulsion will preferably comprisefat droplets between 0.1 and 1.0 μm, particularly 0.1 and 0.5 μm, andhave a pH in the range of 5.5 to 8.0.

[0016] Particularly preferred emulsion compositions are those preparedby mixing an active compound with Intralipid™ or the components thereof(soybean oil, egg phospholipids, glycerol and water).

[0017] Compositions for inhalation or insufflation include solutions andsuspensions in pharmaceutically acceptable, aqueous or organic solvents,or mixtures thereof, and powders. The liquid compositions may containsuitable pharmaceutically acceptable excipients as set out above.Preferably the compositions are administered by the oral or nasalrespiratory route for local or systemic effect. Compositions inpreferably sterile pharmaceutically acceptable solvents may be nebulisedby use of inert gases. Nebulised solutions may be breathed directly fromthe nebulising device, or the nebulising device may be attached to aface mask, tent or intermittent positive pressure breathing machine.Solution, suspension or powder compositions may be administered,preferably orally or nasally, from devices which deliver the formulationin an appropriate manner.

[0018] The anesthetic may be used alone or often in combination withother anesthetics simultaneously administered. Put another way, it willbe appreciated that when using any combination described herein, boththe compound of melatonin or its analogue and the other active agent(s)can be administered to a patient, within a reasonable period of time. Itmay indeed act synergistically with other anesthetic drugs. Thecompounds may be in the same pharmaceutically acceptable carrier andtherefore administered simultaneously. They may be in separatepharmaceutical carriers such as conventional oral dosage forms which aretaken simultaneously. The term “combination” also refers to the casewhere the compounds are provided in separate dosage forms and areadministered sequentially. Therefore, by way of example, one activecompound may be administered as a tablet and then, within a reasonableperiod of time, the second active component may be administered eitheras an oral dosage form such as a tablet or a fast-dissolving oral dosageform. By a “fast dissolving oral formulation” is meant, an oral deliveryform which, when placed on the tongue of a patient, dissolves withinabout 10 seconds.

[0019] The dosage will vary depending upon the deepness of theanesthesia desired, but based upon limited studies to date, it isbelieved that the dosage most effective will be within the range of0.001 mg/kg of body weight to about 500 mg/kg of body weight, morepredictably preferred is the range of 5 mg/kg of body weight to about350 mg/kg of body weight.

[0020] When patients are administered N-acetyl-5-methoxytryptamine(melatonin) or its biologically active analogues, there is a noticeabledecrease in anesthetic hangover. It is believed that this occurs becausemelatonin itself is a naturally-occurring hormone synthesized in thebody by the pineal gland.

[0021] The following examples are offered to further illustrate, but notlimit the invention disclosed herein.

[0022] All experiments were carried out in male Sprague-Dawley rats(300-350 g). Rats were maintained on a 12 hour light/12 hour dark cyclewith free access to food and water. All surgical procedures wereperformed under sterile conditions (skin preparation, sterile fileddrape, gloves, mask, etc.). All instruments and materials were ethyleneoxide sterilized. Non-fasting adult male Sprague Dawley rats (˜300 gm)were anesthetized with halothane in oxygen and weighed. The hair overthe ventral neck and over the back (between the scapulae) was removedwith an electric razor. In the supine position the ventral neck waswashed with povidone-iodine, followed by a 3-cm skin incision, just leftof mid-line. All bleeding points were cauterized. Both the left jugularvein and left common carotid artery were isolated via blunt dissection.The left jugular vein was cannulated with a heparinized (20 U/ml)saline-filled silastic catheter (0.012-in ID, 0.025-in OD) advanced˜3-cm into the right atrium. The jugular catheter was secured to thevein with 4-O silk at the point of insertion, as well as at the rostraljugular ligature.

[0023] After implantation of intravascular catheters, rats were housedin individual stainless steel cages. Studies with melatonin were carriedout 5-7 days after surgery. Crystalline Melatonin powder was obtainedfrom Sigma (Sigma Chemical Co. St. Louis, Mo.).

[0024] The melatonin was prepared for anesthetic use in the followingmanner:

[0025] 100 mg melatonin added to 1 ml of intralipid and 1 ml of Ringer'sLactate (final concentration=50 mg melatonin/ml).

[0026] Results

[0027] Rat 1: 250 mg/kg followed approximately 1 minute later by 65mg/kg: the animal was very drugged but did not lose righting reflex.

[0028] Rat 2: 250 mg/kg resulted in loss of righting and eyelashreflexes and inability to pull his hind paw in response to pressureapplied to it.

[0029] Rat 3: 320 mg/kg resulted in loss of righting and eyelashreflexes and inability to pull his hind paw in response to pressureapplied to it.

[0030] Rat 4: 370 mg/kg resulted in loss of righting and eyelashreflexes and inability to pull his hind paw in response to pressureapplied to it.

[0031] Additional preparation of melatonin occurred with cyclodextrin asfollows:

[0032] 100 mg melatonin added to 1 ml of cyclodextrin 40% and 1 ml ofintralipid (final concentration=50 mg melatonin/ml)

[0033] Results

[0034] Rat 1: 315 mg/kg: the animal moves slowly but no loss of rightingreflex.

[0035] Rat 2: 460 mg/kg resulted in loss of righting reflex.

[0036] Another group of rats received the solvent alone and did notresult in any effect. This demonstrates the anesthetic property ofmelatonin, and that the invention accomplishes its stated objectives.

[0037] Melatonin in 40% cyclodextrin

[0038] Rat 1: 315 mg/kg resulted in ptosis, loss of eye blink responseand loss of paw pinch response to a pressure of 60 mmHg using a very lowprofile load cell (Omega part number LCKD-1KG, measurement range of 0-1kg) from OMEGA Engineering, INC. One Omega Drive, Stamford, Conn.06907-0047. The righting reflex was lost for 27 min.

[0039] Rat 2: 374 mg/kg resulted in ptosis and loss of eye blinkresponse. The righting reflex was lost for 15 min. The animal respondedto a paw pinch response to a pressure of 60 mmHg by pulling his pawwithout vocalization.

[0040] Rat 3: Administration of the solvent (40% cyclodextrin) did notaffect the animal behavior and did not result in sedation or hypnosis.

[0041] In a recent publication of the inventor concerning thisinvention, additional data relating to the invention is disclosed,Anesthesia and Analgesia, 91:473-479(2000), the disclosure of which isincorporated herein by reference.

[0042] The following examples are comparative examples demonstrating theeffectiveness of melatonin for induction of general anesthesia in ratsand how it compares to other known anesthetics in inducing generalanesthesia.

COMPARATIVE EXAMPLES

[0043] The goal of these examples was to determine the doses ofmelatonin, thiopental and propofol needed to induce anesthesia in 50%and 95% of rats and to evaluate the time course of different indices ofanesthesia. Rats were randomly assigned to receive three cumulativedoses of 6.67 mg/kg i.v. thiopental,3.3 mg/kg i.v. propofol or 70 mg/kgi.v. melatonin or three cumulative injections of the vehicle in whichthese drugs were dissolved at intervals of approximately 1 min. Afterthe final cumulative dose, measurements of anesthesia end-points weremade at fixed intervals for an additional 20 minutes. Separate groups ofrats received a single bolus injection of 20 mg/kg i.v. thiopental, 10mg/kg i.v. propofol or 275 mg/kg i.v. of melatonin or the vehicle inwhich these drugs were dissolved. Measurements of anesthesia end-pointswere made in these rats at fixed intervals for 20 minutes. Rightingreflex was scored on a four-point scale (1=immediate/brisk, both feetunder the rat; 2=complete, but slower than normal; 3=slow, feet notplaced under body; and 4=absent). The threshold pressure (mm Hg) atwhich the rat withdrew or vocalized after pinch of one hindpaw wasmeasured. For paw pinch, a subminiature, very low profile load cellOmega part number LCKD-LKG, measurement range of 0-1 kg) (OMEGAEngineering, INC., Stamford, Conn.) was used to measure the amount ofpressure applied to the rat's paw. The action of compressing the loadcell between the faces of the sponge clamp assembly results in a changein impedance within the load cell, and this in turn, was then convertedto mm Hg pressure by the monitor. The presence or absence of eyelashreflex was noted on a three-point scale (1=normal, 2=weak; and3=absent). The strength of grip by the forepaws was determined on afour-point scale (0=absent, 1=weak, 2=moderate and 3=strong).

[0044] Thiopental was purchased from Abbott Laboratories (NorthernChicago, Ill.). Propofol was purchased from Zeneca Pharmaceuticals(Wilmington, Del.). Melatonin was purchased from Sigma Chemical Co. (St.Louis, Mo.). Thiopental and propofol were dissolved in saline andIntralipid™ respectively. Melatonin was dissolved in a mixturecomprising 25% v/v propylene glycol and 25% v/v 1-methyl-2-pyrrolidinonein sterile water. For the cumulative injections, the individual doseswere administered in a volume of 0.2 ml and the maximum volume of druginjected did not exceed 0.6 ml. For the bolus injections, the volume ofdrug injected ranged from 0.6 to 0.75 ml.

[0045] Intravenous injection of saline, Intralipid™ or the vehicle formelatonin did not affect righting reflex, grip strength, or eyelashreflex. Neither saline nor Intralipid™ altered paw withdrawal threshold.However, the vehicle for melatonin produced a significant short-livedincrease in paw withdrawal threshold that subsequently decreased to nearbaseline levels.

[0046] Cumulative i.v. injection of divided doses of thiopental caused aprogressive loss of righting reflex, grip strength and eyelash reflexwith an estimated ED₉₅ (and 95% CI) for the loss of righting reflex of23.8 (15.4-36.7) mg/kg i.v. Bolus injection of 20 mg/kg thiopentalresulted in an immediate loss of righting reflex and grip strength thatwas maximal at 1 min and resolved within 15 min. These effects were notaccompanied by a change in paw withdrawal threshold.

[0047] Cumulative i.v. injection of divided doses of propofol caused aprogressive loss of righting reflex, grip strength and eyelash reflexwith an estimated ED₉₅ (and 95% CI) for loss of righting reflex of 14.9(6.4-34.9) mg/kg. Bolus injection of 10 mg/kg i.v. propofol caused animmediate loss of righting reflex and grip strength that was maximal for5 min and resolved within 10 min. These effects were accomplished by asignificant increase in paw withdrawal threshold of similar duration.

[0048] Cumulative i.v. injection of divided doses of melatonin caused aprogressive loss of righting reflex and grip strength, but did notappreciably blunt the eyelash reflex. It also dose-dependently increasedpaw withdrawal threshold. The estimated b 95 (and 95% CI) of melatoninfor loss of righting reflex was 312 (205-476) mg/kg i.v. Bolus injectionof 275 mg/kg i.v. melatonin resulted in an immediate loss of rightingreflex and grip strength that was maximal for 5 min and resolved to nearbaseline values by 15 min. This dose of melatonin also increased pawwithdrawal threshold as compared to vehicle. The increase in pawwithdrawal threshold persisted for at least 20 min, and was apparent atdoses of 140 mg/kg or greater.

[0049] This data demonstrates that intravenous administration ofmelatonin can induce general anesthesia. This anesthesia is accompaniedby an analgesia that persists after the return of consciousness.

What is claimed is:
 1. An anesthetic composition comprising: apharmaceutically acceptable anesthetic carrier, and an anestheticinducing effective amount of N-acetyl-5-methoxytryptamine, or abiologically active analogue thereof.
 2. The anesthetic composition ofclaim 1 wherein the anesthetic is N-acetyl-5-methoxytryptamine.
 3. Theanesthetic composition of claim 1 wherein the amount ofN-acetyl-5-methoxytryptamine or biologically active analogue thereof issufficient to provide a dose of from about 0.001 mg/kg of body weight toabout 500 mg/kg of body weight.
 4. The anesthetic composition of claim 1wherein the amount of N-acetyl-5-methoxytryptamine or biologicallyactive analogue thereof is sufficient to provide a dose of from about 5mg/kg of body weight to about 350 mg/kg of body weight.
 5. A method ofinducing anesthesia comprising: administering to a patient an anesthesiainducing effective amount of N-acetyl-5-methoxytryptamine or abiologically active analogue thereof.
 6. The method of claim 5 whereinthe anesthetic is N-acetyl-5-methoxytryptamine.
 7. The method of claim 5wherein the amount of N-acetyl-5-methoxytryptamine or biologicallyactive analogue thereof administered is a dose of from about 0.001 mg/kgof body weight to about 500 mg/kg of body weight.
 8. The method of claim5 wherein the amount of N-acetyl-5-methoxytryptamine or biologicallyactive analogue thereof administered is from about 5 mg/kg of bodyweight to about 350 mg/kg of body weight.
 9. The method of claim 5wherein the administering is by a method selected from the groupconsisting of oral administration, nasal respiratory administration,bolus injection, intravenous administration, continuing infusion,rectal, vaginal, sublingual and cutaneous administration.
 10. The methodof claim 9 wherein the administration is by an initial bolus injection,followed by intravenous administration.
 11. The method of claim 5wherein the administration is in combination with simultaneousadministration of another anesthetic.
 12. The method of claim 5 whereinN-acetyl-5-methoxytryptamine or biologically active analogue thereof isderived from the pineal gland.